Owing to the fact that chromium stainless steels low in steel chromium content and also low in nickel content are much more advantageous in terms of price than austenitic stainless steels like SUS304 steel, they are suitable for applications like structural steels that are used in large quantities. Depending on the composition, this kind of chromium steel has a ferritic structure or a martensitic structure, but ferritic and martensitic stainless steels are generally poor in low-temperature toughness and corrosion resistance of welds. For example, in the case of a martensitic stainless steel as typified by SUS410, C content is high at around 0.1 mass %, so that toughness of a weld and workability of a weld are poor, and since welding work efficiency is also poor because preheating is necessary when welding, issues remain regarding application to members that require welding.
As a way for preventing such weld property degradation, methods such as set forth in Patent Document 1 and Patent Document 2 have been proposed which utilize martensitic formation at the weld to prevent loss of corrosion resistance and low-temperature toughness. What Patent Document 1 teaches is a method of including Cr: 10 to 18 mass %, Ni: 0.1 to 3.4 mass %, Si: 1.0 mass % or less and Mn: 4.0 mass % or less and further lowering C to 0.030 mass % or less and N to 0.020 mass % or less in order to form massive martensite at weld heat-affected zones, thus proposing a martensitic stainless steel for welded structures that is improved in weld performance.
Low-chromium stainless steels utilizing this kind of martensite transformation at welds is actually used as a frame material of marine containers and no weld corrosion resistance or low-temperature toughness problems have been reported up to now.
However, it has been learned that cases will arise in which corrosion resistance at the welds is inadequate when utilized in a use environment that is a severe corrosion environment (prolonged wetness duration, high chloride concentration, high temperature, low pH, etc.). For example, cases have been reported of intergranular corrosion occurring at weld heat-affected zones when used in the cargo container or the like of a railway freight car for hauling coal or iron ore. This is because a Cr-depletion layer occurring as the result of Cr carbide precipitation under the effect of heat of multiple welding passes corrodes.
As a method for improving the corrosion resistance of the weld heat-affected zone and toughness of the weld of a low-chromium stainless steel, it is effective to increase the purity of a steel like the foregoing to a high level and additionally add elements for fixing carbon and nitrogen as carbides and nitrides, so that various steels produced using these methods have been reported.
For example, Patent Document 3 teaches use of martensite transformation to prevent degradation of chromium steel intergranular corrosion resistance by addition of suitable amounts of the carbon and nitrogen stabilizing elements Nb and Ti, and also teaches a chromium steel excellent in low-temperature toughness. Similarly, Patent Document 4 teaches a Fe—Cr alloy improved in weld corrosion resistance by addition of the carbide-forming elements Ti, Nb, Ta and Zr. However, this Document requires inclusion of Co, V and W and is aimed at improving initial corrosion resistance.
Although the intergranular corrosion resistance of weld heat-affected zones is improved in a martensitic stainless steel to which Ti, Nb and other stabilizing elements are added, there is a problem of preferential corrosion occurring near the interface (bond area) between the weld metal and the adjacent heat-affected zone having a massive martensite structure.
This phenomenon resembles the phenomenon called knife-line attack that, as taught by Non-patent Document 1, is observed in the welds of SUS321 and SUS347 stabilized austenitic steels. This is a problem that should be resolved because the interface (bond area) between the weld metal and the heat-affected zone preferentially corrodes and the corrosion region expands progressively.
The cause of knife-line attack is that during welding of a stainless steel in which C is fixed as TiC or NbC, the TiC and NbC enter into solid solution at regions with a heat history of having risen to about 1200° C. or greater and Cr carbide precipitates at the grain boundaries to lower corrosion resistance when the steel thereafter passes through the sensitizing temperature range in the cooling process. Patent Document 5 therefore teaches low-chromium stainless steel enabling multipass welding that is excellent in corrosion resistance of heat-affected zone and does not sustain occurrence of knife-line attack even after multiple welding passes, and defines γp (gamma potential), an index for evaluating austenite stability, as 80% or greater, Cr: 10 to 15%, Mn: greater than 1.5 to 2.5%, Ni: 0.2 to 1.5%, and Ti: 4×(C %+N %) or greater. γp=420×Co+470×N %+23×Ni %+9×Cu %+7×Mn %−11.5×Cr %−11.5×Si %−12×Mo %−23×V %−47×Nb %−49×Ti %−52×Al %+189≧80%.
Further, Patent Document 5 teaches prevention of edge cracking during hot rolling by controlling hot-rolling heating temperature to a temperature at which austenite single phase region or delta ferrite amount becomes greater than 50% and prevention of surface defects owing to TiN crystallization by making Ti×N 0.004 or less.
On the other hand, it is known that the surface of the weld heat-affected zone of a low-chromium stainless steel has a problem of corrosion occurring in a form similar to knife-line attack owing to the fact that the oxide scale forms to a greater thickness than in SUS304, SUS430 and the like, so that a Cr-depletion layer is formed directly under the scale, and in Patent Document 5, not only for intergranular corrosion resistance, but also for preventing preferential corrosion occurrence near the weld fusion line, and Cr content of 11.4% or greater at Mn of 1.5 to 2.5% is considered preferable.
However, a study by the present inventors found that preferential corrosion near the weld bond area cannot be prevented at a Mn content of 1.5% or greater unless Cr amount is controlled to 13% or greater.
Further, it was discovered that the preferential corrosion of the weld heat-affected zone near the bond in the steel under consideration is rarely caused by the sensitization arising just after Ti(CN) solution treatment as generally known to be the cause in austenitic stainless steels but is chiefly due to the aforesaid oxidization.
It was found that increasing the Cr content of the base metal to 13% or greater is effective for inhibiting corrosion of the Cr-depletion layer caused by oxidation during welding and that corrosion similar to knife-line attack cannot be adequately prevented in the Cr content range of 10 to 13% common for martensitic stainless steels. On the other hand, increasing Cr content to 13% or greater is hard because it narrows the austenite single phase temperature range, lowers the toughness of the weld heat-affected zone by δ ferrite, and causes loss of heat-affected zone intergranular corrosion resistance. Therefore, a technology has been desired that improves corrosion resistance at a Cr content of 13% or less by inhibiting weld heat-affected zone scale formation.